"We have in the middle basically three decades of problems compounded by high unemployment," said Lawrence Mishel of the Economic Policy Institute, a left-of-center research group in Washington. "That high unemployment we know depresses wage growth throughout the wage scale, but more so for the bottom than the middle and the middle than the top."
In his analysis, Mr. Saez said he saw no reason that the trend would reverse for 2012, which has not yet been analyzed. For that year, the "top 1 percent income will likely surge, due to booming stock prices, as well as retiming of income to avoid the higher 2013 top tax rates," Mr. Saez wrote, referring to income tax increases for the wealthy that were passed by Congress in January. The incomes of the other "99 percent will likely grow much more modestly," he said.
Excluding earnings from investment gains, the top 10 percent of earners took 46.5 percent of all income in 2011, the highest proportion since 1917, Mr. Saez said, citing a large body of work on earnings distribution over the last century that he has produced with the economist Thomas Piketty of the Paris School of Economics.
(Read More: Where the 1% Live)
Concern for the declining wages of working Americans and persistent high levels of inequality featured heavily in President Obama's State of the Union address this week. He proposed raising the federal minimum wage to $9 from $7.25 as one way to ameliorate the trend, a proposal that might lift the earnings of 15 million low-income workers by the end of 2015.
"Let's declare that in the wealthiest nation on Earth, no one who works full time should have to live in poverty," Mr. Obama said in his address to Congress.
Mr. Obama's economic advisers say that he has been animated by the country's yawning levels of inequality, and the administration has put forward several proposals to address the gap. Those include higher taxes on a small group of the wealthiest families and an expansion of aid to lower- and middle-income families through programs like the Affordable Care Act.
The data analyzed by Mr. Piketty and Mr. Saez shows that income inequality — as measured by the proportion of income taken by the top 1 percent of earners — reached a modern high just before the recession hit in 2009. The financial crisis and its aftermath hit wealthy families hard. But since then, their earnings have snapped back, if not to their 2007 peak.
That is not true for average working families. After accounting for inflation, median family income has declined over the last two years. In 2011, it stagnated for the poorest and dropped for those in the middle of the income distribution, census data show. Median household income, which was $50,054 in 2011, is about 9 percent lower than it was in 1999, after accounting for inflation.
Measures of inequality differ depending on whether they are measured after or before taxes, and whether or not they include government transfers like Social Security payments, food stamps and other credits.
(Read More: Personal Services for the 1%)
Research led by the Cornell economist Richard V. Burkhauser, for instance, sought to measure the economic health of middle-class households including income, taxes, transfer programs and benefits like health insurance. It found that from 1979 to 2007, median income grew by about 18.2 percent over all rather than by 3.2 percent counting income alone.
In an interview, Mr. Burkhauser said his numbers measured "how are the resources that person has to live on changing over time," whereas Mr. Piketty and Mr. Saez's numbers measure "how are different people being rewarded in the marketplace."
"That's a fair question to ask, but it's a very different question to ask than, 'What resources do Americans have?' " Mr. Burkhauser said. Notably, many of the Obama administration's progressive policies have been aimed at blunting the effects of income inequality, rather than tackling income inequality itself.
Mr. Saez has advocated much more aggressive policies aimed at income inequality. "Falls in income concentration due to economic downturns are temporary unless drastic regulation and tax policy changes are implemented," Mr. Saez said in his analysis.
The recent policy changes, including tax increases and financial regulatory reform, he wrote, "are not negligible but they are modest relative to the policy changes that took place coming out of the Great Depression. Therefore, it seems unlikely that U.S. income concentration will fall much in the coming years."